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The sun both checks the formation of winds and stimulates it. When
the evaporation is small in amount and faint the sun wastes it and
dissipates by its greater heat the lesser heat contained in the
evaporation. It also dries up the earth, the source of the
evaporation, before the latter has appeared in bulk: just as, when you
throw a little fuel into a great fire, it is often burnt up before
giving off any smoke. In these ways the sun checks winds and
prevents them from rising at all: it checks them by wasting the
evaporation, and prevents their rising by drying up the earth quickly.
Hence calm is very apt to prevail about the rising of Orion and
lasts until the coming of the Etesiae and their 'forerunners'.
Calm is due to two causes. Either cold quenches the evaporation, for
instance a sharp frost: or excessive heat wastes it. In the
intermediate periods, too, the causes are generally either that the
evaporation has not had time to develop or that it has passed away and
there is none as yet to replace it.
Both the setting and the rising of Orion are considered to be
treacherous and stormy, because they place at a change of season
(namely of summer or winter; and because the size of the constellation
makes its rise last over many days) and a state of change is always
indefinite and therefore liable to disturbance.
The Etesiae blow after the summer solstice and the rising of the
dog-star: not at the time when the sun is closest nor when it is
distant; and they blow by day and cease at night. The reason is that
when the sun is near it dries up the earth before evaporation has
taken place, but when it has receded a little its heat and the
evaporation are present in the right proportion; so the ice melts
and the earth, dried by its own heat and that of the sun, smokes and
vapours. They abate at night because the cold pf the nights checks the
melting of the ice. What is frozen gives off no evaporation, nor
does that which contains no dryness at all: it is only where something
dry contains moisture that it gives off evaporation under the
influence of heat.
The question is sometimes asked: why do the north winds which we
call the Etesiae blow continuously after the summer solstice, when
there are no corresponding south winds after the winter solstice?
The facts are reasonable enough: for the so-called 'white south winds'
do blow at the corresponding season, though they are not equally
continuous and so escape observation and give rise to this inquiry.
The reason for this is that the north wind I from the arctic regions
which are full of water and snow. The sun thaws them and so the
Etesiae blow: after rather than at the summer solstice. (For the
greatest heat is developed not when the sun is nearest to the north,
but when its heat has been felt for a considerable period and it has
not yet receded far. The 'bird winds' blow in the same way after the
winter solstice. They, too, are weak Etesiae, but they blow less and
later than the Etesiae. They begin to blow only on the seventieth
day because the sun is distant and therefore weaker. They do not
blow so continuously because only things on the surface of the earth
and offering little resistance evaporate then, the thoroughly frozen
parts requiring greater heat to melt them. So they blow intermittently
till the true Etesiae come on again at the summer solstice: for from
that time onwards the wind tends to blow continuously.) But the
south wind blows from the tropic of Cancer and not from the
antarctic region.
There are two inhabitable sections of the earth: one near our upper,
or nothern pole, the other near the other or southern pole; and
their shape is like that of a tambourine. If you draw lines from the
centre of the earth they cut out a drum-shaped figure. The lines
form two cones; the base of the one is the tropic, of the other the
ever visible circle, their vertex is at the centre of the earth. Two
other cones towards the south pole give corresponding segments of
the earth. These sections alone are habitable. Beyond the tropics no
one can live: for there the shade would not fall to the north, whereas
the earth is known to be uninhabitable before the sun is in the zenith
or the shade is thrown to the south: and the regions below the Bear
are uninhabitable because of the cold.
(The Crown, too, moves over this region: for it is in the zenith
when it is on our meridian.)
So we see that the way in which they now describe the geography of
the earth is ridiculous. They depict the inhabited earth as round, but
both ascertained facts and general considerations show this to be
impossible. If we reflect we see that the inhabited region is
limited in breadth, while the climate admits of its extending all
round the earth. For we meet with no excessive heat or cold in the
direction of its length but only in that of its breadth; so that there
is nothing to prevent our travelling round the earth unless the extent
of the sea presents an obstacle anywhere. The records of journeys by
sea and land bear this out. They make the length far greater than
the breadth. If we compute these voyages and journeys the distance
from the Pillars of Heracles to India exceeds that from Aethiopia to
Maeotis and the northernmost Scythians by a ratio of more than 5 to 3,
as far as such matters admit of accurate statement. Yet we know the
whole breadth of the region we dwell in up to the uninhabited parts:
in one direction no one lives because of the cold, in the other
because of the heat.
But it is the sea which divides as it seems the parts beyond India
from those beyond the Pillars of Heracles and prevents the earth
from being inhabited all round.
Now since there must be a region bearing the same relation to the
southern pole as the place we live in bears to our pole, it will
clearly correspond in the ordering of its winds as well as in other
things. So just as we have a north wind here, they must have a
corresponding wind from the antarctic. This wind cannot reach us since
our own north wind is like a land breeze and does not even reach the
limits of the region we live in. The prevalence of north winds here is
due to our lying near the north. Yet even here they give out and
fail to penetrate far: in the southern sea beyond Libya east and
west winds are always blowing alternately, like north and south
winds with us. So it is clear that the south wind is not the wind that
blows from the south pole. It is neither that nor the wind from the
winter tropic. For symmetry would require another wind blowing from
the summer tropic, which there is not, since we know that only one
wind blows from that quarter. So the south wind clearly blows from the
torrid region. Now the sun is so near to that region that it has no
water, or snow which might melt and cause Etesiae. But because that
place is far more extensive and open the south wind is greater and
stronger and warmer than the north and penetrates farther to the north
than the north wind does to the south.
The origin of these winds and their relation to one another has
now been explained.
6
Let us now explain the position of the winds, their oppositions,
which can blow simultaneously with which, and which cannot, their
names and number, and any other of their affections that have not been
treated in the 'particular questions'. What we say about their
position must be followed with the help of the figure. For
clearness' sake we have drawn the circle of the horizon, which is
round, but it represents the zone in which we live; for that can be
divided in the same way. Let us also begin by laying down that those
things are locally contrary which are locally most distant from one
another, just as things specifically most remote from one another
are specific contraries. Now things that face one another from
opposite ends of a diameter are locally most distant from one another.
(See diagram.)
Let A be the point where the sun sets at the equinox and B, the
point opposite, the place where it rises at the equinox. Let there
be another diameter cutting this at right angles, and let the point
H on it be the north and its diametrical opposite O the south. Let Z
be the rising of the sun at the summer solstice and E its setting at
the summer solstice; D its rising at the winter solstice, and G its
setting at the winter solstice. Draw a diameter from Z to G from D
to E. Then since those things are locally contrary which are most
distant from one another in space, and points diametrically opposite
are most distant from one another, those winds must necessarily be
contrary to one another that blow from opposite ends of a diameter.
The names of the winds according to their position are these.
Zephyrus is the wind that blows from A, this being the point where the
sun sets at the equinox. Its contrary is Apeliotes blowing from B
the point where the sun rises at the equinox. The wind blowing from H,
the north, is the true north wind, called
Aparctias: while Notus
blowing from O is its contrary; for this point is the south and O is
contrary to H, being diametrically opposite to it.
Caecias blows
from Z, where the sun rises at the summer solstice. Its contrary is
not the wind blowing from E but
Lips blowing from G. For Lips blows
from the point where the sun sets at the winter solstice and is
diametrically opposite to Caecias: so it is its contrary.
Eurus
blows from D, coming from the point where the sun rises at the
winter solstice. It borders on Notus, and so we often find that people
speak of 'Euro-
Noti'. Its contrary is not Lips blowing from G but
the wind that blows from E which some call
Argestes, some Olympias,
and some Sciron. This blows from the point where the sun sets at the
summer solstice, and is the only wind that is diametrically opposite
to Eurus. These are the winds that are diametrically opposite to one
another and their contraries.
There are other winds which have no contraries. The wind they call
Thrascias, which lies between Argestes and Aparctias, blows from I;
and the wind called
Meses, which lies between Caecias and Aparctias,
from K. (The line IK nearly coincides with the ever visible circle,
but not quite.) These winds have no contraries. Meses has not, or else
there would be a wind blowing from the point M which is
diametrically opposite. Thrascias corresponding to the point I has
not, for then there would be a wind blowing from N, the point which is
diametrically opposite. (But perhaps a local wind which the
inhabitants of those parts call Phoenicias blows from that point.)
These are the most important and definite winds and these their
places.
There are more winds from the north than from the south. The
reason for this is that the region in which we live lies nearer to the
north. Also, much more water and snow is pushed aside into this
quarter because the other lies under the sun and its course. When this
thaws and soaks into the earth and is exposed to the heat of the sun
and the earth it necessarily causes evaporation to rise in greater
quantities and over a greater space.
Of the winds we have described Aparctias is the north wind in the
strict sense. Thrascias and Meses are north winds too. (Caecias is
half north and half east.) South are that which blows from due south
and Lips. East, the wind from the rising of the sun at the equinox and
Eurus. Phoenicias is half south and half east. West, the wind from the
true west and that called Argestes. More generally these winds are
classified as northerly or southerly. The west winds are counted as
northerly, for they blow from the place of sunset and are therefore
colder; the east winds as southerly, for they are warmer because
they blow from the place of sunrise. So the distinction of cold and
hot or warm is the basis for the division of the winds into
northerly and southerly. East winds are warmer than west winds because
the sun shines on the east longer, whereas it leaves the west sooner
and reaches it later.
Since this is the distribution of the winds it is clear that
contrary winds cannot blow simultaneously. They are diametrically
opposite to one another and one of the two must be overpowered and
cease. Winds that are not diametrically opposite to one another may
blow simultaneously: for instance the winds from Z and from D. Hence
it sometimes happens that both of them, though different winds and
blowing from different quarters, are favourable to sailors making
for the same point.
Contrary winds commonly blow at opposite seasons. Thus Caecias and
in general the winds north of the summer solstice blow about the
time of the spring equinox, but about the
autumn equinox Lips; and
Zephyrus about the summer solstice, but about the winter solstice
Eurus.
Aparctias, Thrascias, and Argestes are the winds that fall on others
most and stop them. Their source is so close to us that they are
greater and stronger than other winds. They bring fair weather most of
all winds for the same reason, for, blowing as they do, from close
at hand, they overpower the other winds and stop them; they also
blow away the
clouds that are forming and leave a clear sky-unless
they happen to be very cold. Then they do not bring fair weather,
but being colder than they are strong they condense the clouds
before driving them away.
Caecias does not bring fair weather because it returns upon
itself. Hence the saying: 'Bringing it on himself as Caecias does
clouds.'
When they cease, winds are succeeded by their neighbours in the
direction of the movement of the sun. For an effect is most apt to
be produced in the neighbourhood of its cause, and the cause of
winds moves with the sun.
Contrary winds have either the same or contrary effects. Thus Lips
and Caecias, sometimes called
Hellespontias, are both rainy gestes and
Eurus are dry: the latter being dry at first and rainy afterwards.
Meses and Aparctias are coldest and bring most snow. Aparctias,
Thrascias, and Argestes bring
hail. Notus, Zephyrus, and Eurus are
hot. Caecias covers the sky with heavy clouds, Lips with lighter ones.
Caecias does this because it returns upon itself and combines the
qualities of
Boreas and Eurus. By being cold it condenses and
gathers the vaporous
air, and because it is easterly it carries with
it and drives before it a great quantity of such matter. Aparctias,
Thrascias, and Argestes bring fair weather for the reason we have
explained before. These winds and Meses are most commonly
accompanied by
lightning. They are cold because they blow from the
north, and lightning is due to cold, being ejected when the clouds
contract. Some of these same bring hail with them for the same reason;
namely, that they cause a sudden condensation.
Hurricanes are commonest in autumn, and next in spring: Aparctias,
Thrascias, and Argestes give rise to them most. This is because
hurricanes are generally formed when some winds are blowing and others
fall on them; and these are the winds which are most apt to fall on
others that are blowing; the reason for which, too, we have
explained before.
The Etesiae veer round: they begin from the north, and become for
dwellers in the west
Thrasciae,
Argestae, and Zephyrus (for Zephyrus
belongs to the north). For dwellers in the east they veer round as far
as Apeliotes.
So much for the winds, their origin and
nature and the properties
common to them all or peculiar to each.
7
We must go on to discuss
earthquakes next, for their cause is akin
to our last subject.
The theories that have been put forward up to the present date are
three, and their authors three men,
Anaxagoras of
Clazomenae, and
before him Anaximenes of
Miletus, and later
Democritus of
Abdera.
Anaxagoras says that the
ether, which naturally moves upwards, is
caught in hollows below the earth and so shakes it, for though the
earth is really all of it equally porous, its surface is clogged up by
rain. This implies that part of the whole sphere is 'above' and part
'below': 'above' being the part on which we live, 'below' the other.
This theory is perhaps too primitive to require refutation. It is
absurd to think of up and down otherwise than as meaning that heavy
bodies move to the earth from every quarter, and light ones, such as
fire, away from it; especially as we see that, as far as our knowledge
of the earth goes, the horizon always changes with a change in our
position, which proves that the earth is convex and spherical. It is
absurd, too, to maintain that the earth rests on the air because of
its size, and then to say that impact upwards from below shakes it
right through. Besides he gives no account of the circumstances
attendant on earthquakes: for not every country or every season is
subject to them.
Democritus says that the earth is full of water and that when a
quantity of rain-water is added to this an earthquake is the result.
The hollows in the earth being unable to admit the excess of water
it forces its way in and so causes an earthquake. Or again, the
earth as it dries draws the water from the fuller to the emptier
parts, and the inrush of the water as it changes its place causes
the earthquake.
Anaximenes says that the earth breaks up when it grows wet or dry,
and earthquakes are due to the fall of these masses as they break
away. Hence earthquakes take place in times of
drought and again of
heavy rain, since, as we have explained, the earth grows dry in time
of drought and breaks up, whereas the rain makes it sodden and
destroys its cohesion.
But if this were the case the earth ought to be found to be
sinking in many places. Again, why do earthquakes frequently occur
in places which are not excessively subject to drought or rain, as
they ought to be on the theory? Besides, on this view, earthquakes
ought always to be getting fewer, and should come to an end entirely
some day: the notion of contraction by packing together implies
this. So this is impossible the theory must be impossible too.
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